Composite reinforcing substructures, such as blade stiffeners, sometimes referred to as blade stringers, are commonly used in marine and aircraft industries. These stringers may be fabricated by combining two or more stiffening members. For example, blade-type stringers can be fabricated by combining two members having L or C cross sectional shapes, back-to-back. These members may be formed by manually hot drape forming multiple composite charges over a mandrel or other tool. After forming, the members are placed back-to-back and co-cured in an autoclave. Fabricating blade stringers using multiple separate charges requires multiple tools, is relatively labor intensive and may add to manufacturing flow times.
Structures formed from composite materials, such as blade stiffeners, are susceptible to subsurface damage following an impact. While a surface of a composite structure with low-energy/low-velocity impact damage may show little or no visible signs of damage, such impacts may cause subsurface damage, such as delamination, which may be difficult to detect without sophisticated analysis. To protect for such non-visible damage states, composite structures may be designed with impact witness treatments, which may lower the impact energy required to generate barely visible impact damage (BVID), such that a corresponding generation of BVID will occur due to significant subsurface damage.
To allow for BVID detection, a treatment that may include glass fibers and a matrix material is applied to those composite structures that are at risk for impact damage, such as blade stiffeners on a wing skin panel of an aircraft. To apply this treatment, blade stiffeners generally require post-cure trim operations at a top of a web and subsequent BVID treatment application involving surface preparation, vacuum bagging, and an additional cure cycle. These additional process steps increase a cost of manufacturing this type of structure.
Accordingly, there is a need for a method for forming a composite structure to enable co-cure BVID treatment to be applied directly after forming the composite structure.